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RFC 2225 - Classical IP and ARP over ATM


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Network Working Group                                        M. Laubach
Request for Comments: 2225                                  Com21, Inc.
Category: Standards Track                                    J. Halpern
Obsoletes: 1626, 1577                          Newbridge Networks, Inc.
                                                             April 1998

                     Classical IP and ARP over ATM

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Table of Contents

   1. ABSTRACT  . . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2. ACKNOWLEDGMENT  . . . . . . . . . . . . . . . . . . . . . . .  2
   3. CONVENTIONS . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4. INTRODUCTION  . . . . . . . . . . . . . . . . . . . . . . . .  3
   5. IP SUBNETWORK CONFIGURATION . . . . . . . . . . . . . . . . .  6
   5.1  Background  . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.2  LIS Configuration Requirements  . . . . . . . . . . . . . .  7
   5.3  LIS Router Additional Configuration . . . . . . . . . . . .  8
   6. IP PACKET FORMAT  . . . . . . . . . . . . . . . . . . . . . .  8
   7. DEFAULT VALUE FOR IP MTU OVER ATM AAL5  . . . . . . . . . . .  9
   7.1  Permanent Virtual Circuits  . . . . . . . . . . . . . . . .  9
   7.2  Switched Virtual Circuits . . . . . . . . . . . . . . . . .  9
   7.3  Path MTU Discovery Required . . . . . . . . . . . . . . . . 11
   8. LIS ADDRESS RESOLUTION SERVICES . . . . . . . . . . . . . . . 11
   8.1  ATM-based ARP and InARP Equivalent Services . . . . . . . . 11
   8.2  Permanent Virtual Connections . . . . . . . . . . . . . . . 12
   8.3  Switched Virtual Connections  . . . . . . . . . . . . . . . 12
   8.4  ATMARP Single Server Operational Requirements . . . . . . . 13
   8.5  ATMARP Client Operational Requirements  . . . . . . . . . . 14
   8.5.1  Client ATMARP Table Aging . . . . . . . . . . . . . . . . 16
   8.5.2  Non-Normal VC Operations  . . . . . . . . . . . . . . . . 17
   8.5.3  Use of ATM ARP in Mobile-IP Scenarios . . . . . . . . . . 17
   8.6  Address Resolution Server Selection . . . . . . . . . . . . 17
   8.6.1  PVCs to ATMARP Servers  . . . . . . . . . . . . . . . . . 18
   8.7  ATMARP Packet Formats . . . . . . . . . . . . . . . . . . . 18

   8.7.1  ATMARP/InATMARP Request and Reply Packet Formats  . . . . 18
   8.7.2  Receiving Unknown ATMARP packets  . . . . . . . . . . . . 20
   8.7.3  TL, ATM Number, and ATM Subaddress Encoding . . . . . . . 20
   8.7.4  ATMARP_NAK Packet Format  . . . . . . . . . . . . . . . . 21
   8.7.5  Variable Length Requirements for ATMARP Packets . . . . . 21
   8.8  ATMARP/InATMARP Packet Encapsulation  . . . . . . . . . . . 22
   9. IP BROADCAST ADDRESS  . . . . . . . . . . . . . . . . . . . . 23
   10. IP MULTICAST ADDRESS . . . . . . . . . . . . . . . . . . . . 23
   11. SECURITY CONSIDERATIONS  . . . . . . . . . . . . . . . . . . 23
   12. MIB SPECIFICATION  . . . . . . . . . . . . . . . . . . . . . 24
   13. OPEN ISSUES  . . . . . . . . . . . . . . . . . . . . . . . . 24
   14. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . 24
   15. AUTHORS' ADDRESSES . . . . . . . . . . . . . . . . . . . . . 26
   APPENDIX A - Update Information  . . . . . . . . . . . . . . . . 27
   FULL COPYRIGHT STATEMENT . . . . . . . . . . . . . . . . . . . . 28

1.  ABSTRACT

   This memo defines an initial application of classical IP and ARP in
   an Asynchronous Transfer Mode (ATM) network environment configured as
   a Logical IP Subnetwork (LIS) as described in Section 5.  This memo
   does not preclude the subsequent development of ATM technology into
   areas other than a LIS; specifically, as single ATM networks grow to
   replace many Ethernet local LAN segments and as these networks become
   globally connected, the application of IP and ARP will be treated
   differently.  This memo considers only the application of ATM as a
   direct replacement for the "wires" and local LAN segments connecting
   IP end-stations ("members") and routers operating in the "classical"
   LAN-based paradigm.  Issues raised by MAC level bridging and LAN
   emulation are beyond the scope of this paper.

   This memo introduces general ATM technology and nomenclature.
   Readers are encouraged to review the ATM Forum and ITU-TS (formerly
   CCITT) references for more detailed information about ATM
   implementation agreements and standards.

2.  ACKNOWLEDGMENT

   The authors would like to thank the efforts of the IP over ATM
   Working Group of the IETF.  Without their substantial, and sometimes
   contentious support, of the Classical IP over ATM model, this updated
   memo would not have been possible.  Section 7, on Default MTU, has
   been incorporated directly from Ran Atkinson's RFC 1626, with his
   permission.  Thanks to Andy Malis for an early review and comments
   for rolc and ion related issues.

3.  CONVENTIONS

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [20].

4.  INTRODUCTION

   The goal of this specification is to allow compatible and
   interoperable implementations for transmitting IP datagrams and ATM
   Address Resolution Protocol (ATMARP) requests and replies over ATM
   Adaptation Layer 5 (AAL5)[2,6].

   This memo specifies the stable foundation baseline operational model
   which will always be available in IP and ARP over ATM
   implementations.  Subsequent memos will build upon and refine this
   model.  However, in the absence or failure of those extensions,
   operations will default to the specifications contained in this memo.
   Consequently, this memo will not reference these other extensions.

   This memo defines only the operation of IP and address resolution
   over ATM, and is not meant to describe the operation of ATM networks.
   Any reference to virtual connections, permanent virtual connections,
   or switched virtual connections applies only to virtual channel
   connections used to support IP and address resolution over ATM, and
   thus are assumed to be using AAL5.  This memo places no restrictions
   or requirements on virtual connections used for other purposes.

   Initial deployment of ATM provides a LAN segment replacement for:

   1)  Local area networks (e.g., Ethernets, Token Rings and FDDI).

   2)  Local-area backbones between existing (non-ATM) LANs.

   3)  Dedicated circuits or frame relay PVCs between IP routers.

   NOTE: In 1), local IP routers with one or more ATM interfaces will be
   able to connect islands of ATM networks.  In 3), public or private
   ATM Wide Area networks will be used to connect IP routers, which in
   turn may or may not connect to local ATM networks.  ATM WANs and LANs
   may be interconnected.

   Private ATM networks (local or wide area) will use the private ATM
   address structure specified in the ATM Forum UNI 3.1 specification
   [9] or as in the ATM Forum UNI 4.0 specification [19].  This
   structure is modeled after the format of an OSI Network Service
   Access Point Address (NSAPA).  A private ATM address uniquely
   identifies an ATM endpoint.

   Public networks will use either the address structure specified in
   ITU-TS recommendation E.164 or the private network ATM address
   structure.  An E.164 address uniquely identifies an interface to a
   public network.

   The characteristics and features of ATM networks are different than
   those found in LANs:

   o   ATM provides a Virtual Connection (VC) switched environment.  VC
       setup may be done on either a Permanent Virtual Connection (PVC)
       or dynamic Switched Virtual Connection (SVC) basis.  SVC call
       management signalling is performed via implementations of the UNI
       3.1 protocol [7,9].

   o   Data to be passed by a VC is segmented into 53 octet quantities
       called cells (5 octets of ATM header and 48 octets of data).

   o   The function of mapping user Protocol Data Units (PDUs) into the
       information field of the ATM cell and vice versa is performed in
       the ATM Adaptation Layer (AAL).  When a VC is created a specific
       AAL type is associated with the VC.  There are four different AAL
       types, which are referred to individually as "AAL1", "AAL2",
       "AAL3/4", and "AAL5".  (NOTE: this memo concerns itself with the
       mapping of IP and ATMARP over AAL5 only.  The other AAL types are
       mentioned for introductory purposes only.)  The AAL type is known
       by the VC end points via the call setup mechanism and is not
       carried in the ATM cell header.  For PVCs the AAL type is
       administratively configured at the end points when the Connection
       (circuit) is set up.  For SVCs, the AAL type is communicated
       along the VC path via UNI 3.1 as part of call setup establishment
       and the end points use the signaled information for
       configuration.  ATM switches generally do not care about the AAL
       type of VCs.  The AAL5 format specifies a packet format with a
       maximum size of (64K - 1) octets of user data.  Cells for an AAL5
       PDU are transmitted first to last, the last cell indicating the
       end of the PDU.  ATM standards guarantee that on a given VC, cell
       ordering is preserved end-to-end.  NOTE: AAL5 provides a non-
       assured data transfer service - it is up to higher-level
       protocols to provide retransmission.

   o   ATM Forum signaling defines point-to-point and point-to-
       point Connection setup [9, 19.]  Multipoint-to-multipoint not yet
       specified by ITU-TS or ATM Forum.

       An ATM Forum ATM address is either encoded as an NSAP form ATM
       EndSystem Address (AESA) or is an E.164 Public-UNI address [9,
       19].  In some cases, both an AESA and an E.164 Public UNI address
       are needed by an ATMARP client to reach another host or router.

       Since the use of AESAs and E.164 public UNI addresses by ATMARP
       are analogous to the use of Ethernet addresses, the notion of
       "hardware address" is extended to encompass ATM addresses in the
       context of ATMARP, even though ATM addresses need not have
       hardware significance.  ATM Forum NSAP format addresses (AESA)
       use the same basic format as U.S. GOSIP OSI NSAPAs [11].  NOTE:
       ATM Forum addresses should not be construed as being U.S. GOSIP
       NSAPAs.  They are not, the administration is different, which
       fields get filled out are different, etc.  However, in this
       document, these will be referred to as NSAPAs.

   This memo describes the initial deployment of ATM within "classical"
   IP networks as a direct replacement for local area networks
   (Ethernets) and for IP links which interconnect routers, either
   within or between administrative domains.  The "classical" model here
   refers to the treatment of the ATM host adapter as a networking
   interface to the IP protocol stack operating in a LAN-based paradigm.

   Characteristics of the classical model are:

   o   The same maximum transmission unit (MTU) size is the default for
       all VCs in a LIS.  However, on a VC-by-VC point-to-point basis,
       the MTU size may be negotiated during connection setup using Path
       MTU Discovery to better suit the needs of the cooperating pair of
       IP members or the attributes of the communications path.  (Refer
       to Section 7.3)

   o   Default LLC/SNAP encapsulation of IP packets.

   o   End-to-end IP routing architecture stays the same.

   o   IP addresses are resolved to ATM addresses by use of an ATMARP
       service within the LIS - ATMARPs stay within the LIS.  From a
       client's perspective, the ATMARP architecture stays faithful to
       the basic ARP model presented in [3].

   o   One IP subnet is used for many hosts and routers.  Each VC
       directly connects two IP members within the same LIS.

   Future memos will describe the operation of IP over ATM when ATM
   networks become globally deployed and interconnected.

   The deployment of ATM into the Internet community is just beginning
   and will take many years to complete.  During the early part of this
   period, we expect deployment to follow traditional IP subnet
   boundaries for the following reasons:

   o   Administrators and managers of IP subnetworks will tend to
       initially follow the same models as they currently have deployed.
       The mindset of the community will change slowly over time as ATM
       increases its coverage and builds its credibility.

   o   Policy administration practices rely on the security, access,
       routing, and filtering capability of IP Internet gateways: i.e.,
       firewalls.  ATM will not be allowed to "back-door" around these
       mechanisms until ATM provides better management capability than
       the existing services and practices.

   o   Standards for global IP over ATM will take some time to complete
       and deploy.

   This memo details the treatment of the classical model of IP and
   ATMARP over ATM.  This memo does not preclude the subsequent
   treatment of ATM networks within the IP framework as ATM becomes
   globally deployed and interconnected; this will be the subject of
   future documents.  This memo does not address issues related to
   transparent data link layer interoperability.

5.  IP SUBNETWORK CONFIGURATION

5.1 Background

   In the LIS scenario, each separate administrative entity configures
   its hosts and routers within a LIS.  Each LIS operates and
   communicates independently of other LISs on the same ATM network.

   In the classical model, hosts communicate directly via ATM to other
   hosts within the same LIS using the ATMARP service as the mechanism
   for resolving target IP addresses to target ATM endpoint addresses.
   The ATMARP service has LIS scope only and serves all hosts in the
   LIS.  Communication to hosts located outside of the local LIS is
   provided via an IP router.  This router is an ATM endpoint attached
   to the ATM network that is configured as a member of one or more
   LISs.  This configuration MAY result in a number of disjoint LISs
   operating over the same ATM network.  Using this model hosts of
   differing IP subnets MUST communicate via an intermediate IP router
   even though it may be possible to open a direct VC between the two IP
   members over the ATM network.

   By default, the ATMARP service and the classical LIS routing model
   MUST be available to any IP member client in the LIS.

5.2 LIS Configuration Requirements

   The requirements for IP members (hosts, routers) operating in an ATM
   LIS configuration are:

   o   All members of the LIS have the same IP network/subnet number and
       address mask [8].

   o   All members within a LIS are directly connected to the ATM
       network.

   o   All members of a LIS MUST have a mechanism for resolving IP
       addresses to ATM addresses via ATMARP (based on [3]) and vice
       versa via InATMARP (based on [12]) when using SVCs.  Refer to
       Section 8 "LIS ADDRESS RESOLUTION SERVICES" in this memo.

   o   All members of a LIS MUST have a mechanism for resolving VCs to
       IP addresses via InATMARP (based on [12]) when using PVCs.  Refer
       to Section 8 "LIS ADDRESS RESOLUTION SERVICES" in this memo.

   o   All members within a LIS MUST be able to communicate via ATM with
       all other members in the same LIS; i.e., the Virtual Connection
       topology underlying the intercommunication among the members is
       fully meshed.

   The following list identifies the set of ATM specific parameters that
   MUST be implemented in each IP station connected to the ATM network:

   o   ATM Hardware Address (atm$ha).  The ATM address of the individual
       IP station.

   o   ATMARP Request Address list (atm$arp-req-list): atm$arp-req-list
       is a list containing one or more ATM addresses of individual
       ATMARP servers located within the LIS.  In an SVC environment,
       ATMARP servers are used to resolve target IP addresses to target
       ATM address via an ATMARP request and reply protocol.  ATMARP
       servers MUST have authoritative responsibility for resolving
       ATMARP requests of all IP members using SVCs located within the
       LIS.

   A LIS MUST have a single ATMARP service entry configured and
   available to all members of the LIS who use SVCs.

   In the case where there is only a single ATMARP server within the
   LIS, then all ATMARP clients MUST be configured identically to have
   only one non-null entry in atm$arp-req-list configured with the same
   address of the single ATMARP service.

   If the IP member is operating with PVCs only, then atm$arp-req-list
   MUST be configured with all null entries and the client MUST not make
   queries to either address resolution service.

   Within the restrictions mentioned above and in Section 8, local
   administration MUST decide which server address(es) are appropriate
   for atm$arp-req-list.

   By default, atm$arp-req-list MUST be configured using the MIB [18].

   Manual configuration of the addresses and address lists presented in
   this section is implementation dependent and beyond the scope of this
   document; i.e., this memo does not require any specific configuration
   method.  This memo does require that these addresses MUST be
   configured completely on the client, as appropriate for the LIS,
   prior to use by any service or operation detailed in this memo.

5.3 LIS Router Additional Configuration

   It is RECOMMENDED that routers providing LIS functionality over the
   ATM network also support the ability to interconnect multiple LISs.
   Routers that wish to provide interconnection of differing LISs MUST
   be able to support multiple sets of these parameters (one set for
   each connected LIS) and be able to associate each set of parameters
   to a specific IP network/ subnet number.  In addition, it is
   RECOMMENDED that a router be able to provide this multiple LIS
   support with a single physical ATM interface that may have one or
   more individual ATM endpoint addresses.   NOTE: this does not
   necessarily mean different End System Identifiers (ESIs) when NSAPAs
   are used.  The last octet of an NSAPA is the NSAPA Selector (SEL)
   field which can be used to differentiate up to 256 different LISs for
   the same ESI.  (Refer to Section 5.1.3.1, "Private Networks" in [9].)

6.  IP PACKET FORMAT

   Implementations MUST support IEEE 802.2 LLC/SNAP encapsulation as
   described in [2].  LLC/SNAP encapsulation is the default packet
   format for IP datagrams.

   This memo recognizes that other encapsulation methods may be used
   however, in the absence of other knowledge or agreement, LLC/SNAP
   encapsulation is the default.

   This memo recognizes that end-to-end signaling within ATM may allow
   negotiation of encapsulation method on a per-VC basis.

7.  DEFAULT VALUE FOR IP MTU OVER ATM AAL5

   Protocols in wide use throughout the Internet, such as the Network
   File System (NFS), currently use large frame sizes (e.g., 8 KB).
   Empirical evidence with various applications over the Transmission
   Control Protocol (TCP) indicates that larger Maximum Transmission
   Unit (MTU) sizes for the Internet Protocol (IP) tend to give better
   performance.  Fragmentation of IP datagrams is known to be highly
   undesirable [16].  It is desirable to reduce fragmentation in the
   network and thereby enhance performance by having the IP Maximum
   Transmission Unit (MTU) for AAL5 be reasonably large.  NFS defaults
   to an 8192 byte frame size.  Allowing for RPC/XDR, UDP, IP, and LLC
   headers, NFS would prefer a default MTU of at least 8300 octets.
   Routers can sometimes perform better with larger packet sizes because
   most of the performance costs in routers relate to "packets handled"
   rather than "bytes transferred".  So, there are a number of good
   reasons to have a reasonably large default MTU value for IP over ATM
   AAL5.

   RFC 1209 specifies the IP MTU over SMDS to be 9180 octets, which is
   larger than 8300 octets but still in the same range [1].  There is no
   good reason for the default MTU of IP over ATM AAL5 to be different
   from IP over SMDS, given that they will be the same magnitude.
   Having the two be the same size will be helpful in interoperability
   and will also help reduce incidence of IP fragmentation.

   Therefore, the default IP MTU for use with ATM AAL5 shall be 9180
   octets.  All implementations compliant and conformant with this
   specification shall support at least the default IP MTU value for use
   over ATM AAL5.

7.1  Permanent Virtual Circuits

   Implementations which only support Permanent Virtual Circuits (PVCs)
   will (by definition) not implement any ATM signalling protocol.  Such
   implementations shall use the default IP MTU value of 9180 octets
   unless both parties have agreed in advance to use some other IP MTU
   value via some mechanism not specified here.

7.2  Switched Virtual Circuits

   Implementations that support Switched Virtual Circuits (SVCs) MUST
   attempt to negotiate the AAL CPCS-SDU size using the ATM signalling
   protocol.  The industry standard ATM signalling protocol uses two
   different parts of the Information Element named "AAL Parameters" to
   exchange information on the MTU over the ATM circuit being setup [9].
   The Forward Maximum CPCS-SDU Size field contains the value over the
   path from the calling party to the called party.  The Backwards

   Maximum CPCS-SDU Size Identifier field contains the value over the
   path from the called party to the calling party.  The ATM Forum
   specifies the valid values of this identifier as 1 to 65535
   inclusive.  Note that the ATM Forum's User-to-Network-Interface (UNI)
   signalling permits the MTU in one direction to be different from the
   MTU in the opposite direction, so the Forward Maximum CPCS-SDU Size
   Identifier might have a different value from the Backwards Maximum
   CPCS-SDU Size Identifier on the same connection.

   If the calling party wishes to use the default MTU it shall still
   include the "AAL Parameters" information element with the default
   values for the Maximum CPCS-SDU Size as part of the SETUP message of
   the ATM signalling protocol [9].  If the calling party desires to use
   a different value than the default, it shall include the "AAL
   Parameters" information element with the desired value for the
   Maximum CPCS-SDU Size as part of the SETUP message of the ATM
   Signalling Protocol.  The called party will respond using the same
   information elements and identifiers in its CONNECT message response
   [9].

   If the called party receives a SETUP message containing the "Maximum
   CPCS-SDU Size" in the AAL Parameters information element, it shall
   handle the Forward and Backward Maximum CPCS-SDU Size Identifier as
   follows:

   a)  If it is able to accept the ATM MTU values proposed by the SETUP
       message, it shall include an AAL Parameters information element
       in its response.  The Forward and Backwards Maximum CPCS-SDU Size
       fields shall be present and their values shall be equal to the
       corresponding values in the SETUP message.

   b)  If it wishes a smaller ATM MTU size than that proposed, then it
       shall set the values of the Maximum CPCS-SDU Size in the AAL
       Parameters information elements equal to the desired value in the
       CONNECT message responding to the original SETUP message.

   c)  If the calling endpoint receives a CONNECT message that does not
       contain the AAL Parameters Information Element, but the
       corresponding SETUP message did contain the AAL Parameters
       Information element (including the forward and backward CPCS-SDU
       Size fields), it shall clear the call with cause "AAL Parameters
       cannot be supported".

   d)  If either endpoint receives a STATUS message with cause
       "Information Element Non-existent or Not Implemented" or cause
       "Access Information Discarded", and with a diagnostic field

       indicating the AAL Parameters Information Element identifier, it
       shall clear the call with cause "AAL Parameters cannot be
       supported."

   e)  If either endpoint receives CPCS-SDUs in excess of the negotiated
       MTU size, it may use IP fragmentation or may clear the call with
       cause "AAL Parameters cannot be supported".  In this case, an
       error has occurred either due to a fault in an end system or in
       the ATM network.  The error should be noted by ATM network
       management for human examination and intervention.

   If the called endpoint incorrectly includes the Forward and Backward
   Maximum CPCS-SDU Size fields in the CONNECT messages (e.g., because
   the original SETUP message did not include these fields) or it sets
   these fields to an invalid value, then the calling party shall clear
   the call with cause "Invalid Information Element Contents".

7.3  Path MTU Discovery Required

   The Path MTU Discovery mechanism is Internet Standard RFC 1191 [17]
   and is an important mechanism for reducing IP fragmentation in the
   Internet.  This mechanism is particularly important because new
   subnet ATM uses a default MTU sizes significantly different from
   older subnet technologies such as Ethernet and FDDI.

   In order to ensure good performance throughout the Internet and also
   to permit IP to take full advantage of the potentially larger IP
   datagram sizes supported by ATM, all router implementations that
   comply or conform with this specification must also implement the IP
   Path MTU Discovery mechanism as defined in RFC 1191 and clarified by
   RFC 1435 [14].  Host implementations should implement the IP Path MTU
   Discovery mechanism as defined in RFC 1191.

8.  LIS ADDRESS RESOLUTION SERVICES

8.1 ATM-based ARP and InARP Equivalent Services

   Address resolution within an ATM LIS SHALL make use of the ATM
   Address Resolution Protocol (ATMARP) (based on [3]) and the Inverse
   ATM Address Resolution Protocol (InATMARP) (based on [12]) and as
   defined in this memo.  ATMARP is the same protocol as the ARP
   protocol presented in [3] with extensions needed to support address
   resolution in a unicast server ATM environment.  InATMARP is the same
   protocol as the original InARP protocol presented in [12] but applied
   to ATM networks.  All IP stations MUST support these protocols as
   updated and extended in this memo.  Use of these protocols differs
   depending on whether PVCs or SVCs are used.

8.2 Permanent Virtual Connections

   An IP station MUST have a mechanism (e.g., manual configuration) for
   determining what PVCs it has, and in particular which PVCs are being
   used with LLC/SNAP encapsulation.  The details of the mechanism are
   beyond the scope of this memo.

   All IP members supporting PVCs are required to use the Inverse ATM
   Address Resolution Protocol (InATMARP) (refer to [12]) on those VCs
   using LLC/SNAP encapsulation.  In a strict PVC environment, the
   receiver SHALL infer the relevant VC from the VC on which the
   InATMARP_Request or response InATMARP_Reply was received.  When the
   ATM source and/or target address is unknown, the corresponding ATM
   address length in the InATMARP packet MUST be set to zero (0)
   indicating a null length, and no storage be allocated in the InATMARP
   packet, otherwise the appropriate address field should be filled in
   and the corresponding length set appropriately.  InATMARP packet
   format details are presented later in this memo.

   Directly from [12]: "When the requesting station receives the
   In[ATM]ARP_Reply, it may complete the [ATM]ARP table entry and use
   the provided address information.  NOTE: as with [ATM]ARP,
   information learned via In[ATM]ARP may be aged or invalidated under
   certain circumstances." IP stations supporting PVCs MUST re-validate
   ATMARP table entries as part of the table aging process.  See the
   Section 8.5.1 "Client ATMARP Table Aging".

   If a client has more than one IP address within the LIS and if using
   PVCs, when an InATMARP_Request is received an InATMARP_Reply MUST be
   generated for each such address.

8.3 Switched Virtual Connections

   SVCs require support from address resolution services for resolving
   target IP addresses to target ATM endpoint addresses.  All members in
   the LIS MUST use the same service.  This service MUST have
   authoritative responsibility for resolving the ATMARP requests of all
   IP members within the LIS.

   ATMARP servers do not actively establish connections.  They depend on
   the clients in the LIS to initiate connections for the ATMARP
   registration procedure and for transmitting ATMARP requests.  An
   individual client connects to the ATMARP server using a point-to-
   point LLC/SNAP VC.  The client sends normal ATMARP request packets to
   the server.  The ATMARP server examines each ATMARP_Request packet
   for

   the source protocol and source hardware address information of the
   sending client and uses this information to build its ATMARP table
   cache.  This information is used to generate replies to any ATMARP
   requests it receives.

   InATMARP_Request packets MUST specify valid address information for
   ATM source number, ATM target number, and source protocol address;
   i.e., these fields MUST be non-null in InATMARP_Request packets.

   This memo defines the address resolution service in the LIS and
   constrains it to consist of a single ATMARP server.  Client-server
   interaction is defined by using a single server approach as a
   reference model.

   This memo recognizes the future development of standards and
   implementations of multiple-ATMARP-server models that will extend the
   operations as defined in this memo to provide a highly reliable
   address resolution service.

8.4 ATMARP Single Server Operational Requirements

   A single ATMARP server accepts ATM calls/connections from other ATM
   end points.  After receiving any ATMARP_Request, the server will
   examine the source and target address information in the packet and
   make note of the VC on which the ATMARP_Request arrived.  It will use
   this information as necessary to build and update its ATMARP table
   entries.

   For each ATMARP_Request, then:

   1.  If the source IP protocol address is the same as the target IP
       protocol address and a table entry exists for that IP address and
       if the source ATM hardware address does not match the table entry
       ATM address and there is an open VC associated with that table
       entry that is not the same as the VC associated with the
       ATMARP_Request, the server MUST return the table entry
       information in the ATMARP_Reply, and MUST raise a "duplicate IP
       address detected" condition to the server's management.  The
       table entry is not updated.

   2.  Otherwise, if the source IP protocol address is the same as the
       target IP protocol address, and either there is no table entry
       for that IP address, or a table entry exists for that IP address
       and there is no open VC associated with that table entry, or if
       the VC associated with that entry is the same as the VC for the
       ATMARP_Request, the server MUST either create a new entry or
       update the old entry as appropriate and return that table entry
       information in the ATMARP Reply.

   3.  Otherwise, when the source IP protocol address does not match the
       target IP protocol address, the ATMARP server will generate the
       corresponding ATMARP_Reply if it has an entry for the target
       information in its ATMARP table.  Otherwise, it will generate a
       negative ATMARP reply (ATMARP_NAK).

   4.  Additionally, when the source IP protocol address does not match
       the target IP protocol address and when the server receives an
       ATMARP_Request over a VC, where the source IP and ATM address do
       not have a corresponding table entry, the ATMARP server MUST
       create a new table entry for the source information.
       Explanation: this allows old RFC 1577 clients to register with
       this ATMARP service by just issuing requests to it.

   5.  Additionally, when the source IP protocol address does not match
       the target IP protocol address and where the source IP and ATM
       addresses match the association already in the ATMARP table and
       the ATM address matches that associated with the VC, the server
       MUST update the table timeout on the source ATMARP table entry
       but only if it has been more than 10 minutes since the last
       update.  Explanation: if the client is sending ATMARP requests to
       the server over the same VC that it used to register its ATMARP
       entry, the server should examine the ATMARP request and note that
       the client is still "alive" by updating the timeout on the
       client's ATMARP table entry.

   6.  Additionally, when the source IP protocol address does not match
       the target IP protocol address and where the source IP and ATM
       addresses do not match the association already in the ATMARP
       table, the server MUST NOT update the ATMARP table entry.

   An ATMARP server MUST have knowledge of any open VCs it has and their
   association with an ATMARP table entry, and in particular, which VCs
   support LLC/SNAP encapsulation.  In normal operation, active ATMARP
   clients will revalidate their entries prior to the server aging
   process taking effect.

   Server ATMARP table entries are valid for 20 minutes.  If an entry
   ages beyond 20 minutes without being updated (refreshed) by the
   client, that entry is deleted from the table regardless of the state
   of any VCs that may be associated with that entry.

8.5 ATMARP Client Operational Requirements

   The ATMARP client is responsible for contacting the ATMARP service to
   both initially register and subsequently refresh its own ATMARP
   information.

   The client is also responsible for using the ATMARP service to gain
   and revalidate ATMARP information about other IP members in the LIS
   (server selection overview is discussed in Section 8.6).  As noted in
   Section 5.2, ATMARP clients MUST be configured with the ATM address
   of the appropriate server prior to client ATMARP operation.

   IP clients MUST register their ATM endpoint address with their ATMARP
   server using the ATM address structure appropriate for their ATM
   network connection: i.e., LISs implemented over ATM LANs following
   ATM Forum UNI 3.1 should register using Structure 1; LISs implemented
   over an E.164 "public" ATM network should register using Structure 2.
   A LIS implemented over a combination of ATM LANs and public ATM
   networks may need to register using Structure 3.  Implementations
   based on this memo MUST support all three ATM address structures.
   See Section 8.7.1 for more details regarding the ATMARP Request
   packet format.

   To handle the case when a client has more than one IP address within
   a LIS, when using an ATMARP server, the client MUST register each
   such address.

   For initial registration and subsequent refreshing of its own
   information with the ATMARP service, clients MUST:

   1.  Establish an LLC/SNAP VC connection to a server in the ATMARP
       service for the purposes of transmitting and receiving ATMARP
       packets.

       NOTE: in the case of refreshing its own information with the
       ATMARP service, a client MAY reuse an existing established
       connection to the ATMARP service provided that the connection was
       previously used either to initially register its information with
       the ATMARP service or to refresh its information with the ATMARP
       service.

   2.  After establishing a successful connection to the ATMARP service,
       the client MUST transmit an ATMARP_Request packet, requesting a
       target ATM address for its own IP address as the target IP
       protocol address.  The client checks the ATMARP_Reply and if the
       source hardware and protocol addresses match the respective
       target hardware and protocol addresses, the client is registered
       with the ATMARP service.  If the addresses do not match, the
       client MAY take action, raise alarms, etc.; however, these
       actions are beyond the scope of this memo.  In the case of a
       client having more than one IP address in the list, this step
       MUST be repeated for each IP address.

   3.  Clients MUST respond to ATMARP_Request and InATMARP_Request
       packets received on any VC appropriately.  (Refer to Section 7,
       "Protocol Operation" in RFC 1293 [12].)

       NOTE: for reasons of robustness, clients MUST respond to
       ATMARP_Requests.

   4.  Generate and transmit address resolution request packets to the
       address resolution service.  Respond to address resolution reply
       packets appropriately to build/refresh its own client ATMARP
       table entries.

   5.  Generate and transmit InATMARP_Request packets as needed and
       process InATMARP_Reply packets appropriately.  InATMARP_Reply
       packets should be used to build/refresh its own client ATMARP
       table entries.  (Refer to Section 7, "Protocol Operation" in
       [12].)  If a client has more than one IP address within the LIS
       when an InATMARP_Request is received an InATMARP_Reply MUST be
       generated for each such address.

   The client MUST refresh its ATMARP information with the server at
   least once every 15 minutes.  This is done by repeating steps 1 and
   2.

   An ATMARP client MUST have knowledge of any open VCs it has
   (permanent or switched), their association with an ATMARP table
   entry, and in particular, which VCs support LLC/SNAP encapsulation.

8.5.1 Client ATMARP Table Aging

   Client ATMARP table entries are valid for a maximum time of 15
   minutes.

   When an ATMARP table entry ages, an ATMARP client MUST invalidate the
   table entry.  If there is no open VC server associated with the
   invalidated entry, that entry is deleted.  In the case of an
   invalidated entry and an open VC, the client MUST revalidate the
   entry prior to transmitting any non address resolution traffic on
   that VC; this requirement applies to both PVCs and SVCs.  NOTE: the
   client is permitted to revalidate an ATMARP table entry before it
   ages, thus restarting the aging time when the table entry is
   successfully revalidated.  The client MAY continue to use the open
   VC, as long as the table entry has not aged, while revalidation is in
   progress.

   In the case of an open PVC, the client revalidates the entry by
   transmitting an InATMARP_Request and updating the entry on receipt of
   an InATMARP_Reply.

   In the case of an open SVC, the client revalidates the entry by
   querying the address resolution service.  If a valid reply is
   received (e.g., ATMARP_Reply), the entry is updated.  If the address
   resolution service cannot resolve the entry (i.e., "host not found"),
   the SVC should be closed and the associated table entry removed.  If
   the address resolution service is not available (i.e., "server
   failure") and if the SVC is LLC/SNAP encapsulated, the client MUST
   attempt to revalidate the entry by transmitting an InATMARP_Request
   on that VC and updating the entry on receipt of an InATMARP_Reply.
   If the InATMARP_Request attempt fails to return an InATMARP_Reply,
   the SVC should be closed and the associated table entry removed.

   If a VC with an associated invalidated ATMARP table entry is closed,
   that table entry is removed.

8.5.2 Non-Normal VC Operations

   The specific details on client procedures for detecting non-normal VC
   connection establishment or closures, or failed communications on an
   established VC are beyond the scope of this memo.  It is REQUIRED
   however, that the client MUST remove the associated ATMARP entry for
   a VC that fails to operate properly, as defined by the client, when
   the client closes that VC, when it releases its resources for a VC,
   or prior to any attempt to reopen that VC.  This behavior
   specifically REQUIRES that the client MUST refresh its ATMARP table
   information prior to any attempt to re-establish communication to an
   IP member after a non-normal communications problem has previously
   occurred on a VC to that IP member.

8.5.3 Use of ATMARP In Mobile-IP Scenarios

   When an ATM LIS is used as the home network in a mobile-IP scenario,
   it is RECOMMENDED that the home agent NOT maintain long term
   connections with the ATMARP service.  The absence of this VC will
   permit a mobile node's registration, upon its return to the home
   network, to immediately preempt the home agent's previous gratuitous
   registration.

8.6 Address Resolution Server Selection

   If the client supports PVCs only, the ATMARP server list is empty and
   the client MUST not generate any address resolution requests other
   than the InATMARP requests on a PVC needed to validate that PVC.

   If the client supports SVCs, then the client MUST have a non-NULL
   atm$arp-req-list pointing to the ATMARP server(s) which provides
   ATMARP service for the LIS.

   The client MUST register with a server from atm$arp-req-list.

   The client SHALL attempt to communicate with any of the servers until
   a successful registration is accomplished.  The order in which client
   selects servers to attempt registration, is a local matter, as are
   the number of retries and timeouts for such attempts.

8.6.1 PVCs to ATMARP Servers

   In a mixed PVC and SVC LIS environment, an ATMARP client MAY have a
   PVC to an ATMARP server.  In this case, this PVC is used for ATMARP
   requests and responses as if it were an established SVC.  NOTE: if
   this PVC is to be used for IP traffic, then the ATMARP server MUST be
   prepared to accept and respond appropriately to InATMARP traffic.

8.7 ATMARP Packet Formats

   Internet addresses are assigned independently of ATM addresses.  Each
   host implementation MUST know its own IP and ATM address(es) and MUST
   respond to address resolution requests appropriately.  IP members
   MUST also use ATMARP and InATMARP to resolve IP addresses to ATM
   addresses when needed.

   NOTE: the ATMARP packet format presented in this memo is general in
   nature in that the ATM number and ATM subaddress fields SHOULD map
   directly to the corresponding UNI 3.1 fields used for ATM
   call/connection setup signalling messages.  The IP over ATM Working
   Group expects ATM Forum NSAPA numbers (Structure 1) to predominate
   over E.164 numbers (Structure 2) as ATM endpoint identifiers within
   ATM LANs.  The ATM Forum's VC Routing specification is not complete
   at this time and therefore its impact on the operational use of ATM
   Address Structure 3 is undefined.  The ATM Forum will be defining
   this relationship in the future.  It is for this reason that IP
   members need to support all three ATM address structures.

8.7.1 ATMARP/InATMARP Request and Reply Packet Formats

   The ATMARP and InATMARP request and reply protocols use the same
   hardware type (ar$hrd), protocol type (ar$pro), and operation code
   (ar$op) data formats as the ARP and InARP protocols [3,12].  The
   location of these three fields within the ATMARP packet are in the
   same byte position as those in ARP and InARP packets.  A unique
   hardware type value has been assigned for ATMARP.  In addition,
   ATMARP makes use of an additional operation code for ARP_NAK.  The
   remainder of the ATMARP/InATMARP packet format is different than the
   ARP/InARP packet format.

   The ATMARP and InATMARP protocols have several fields that have the
   following format and values:

   Data:
     ar$hrd   16 bits  Hardware type
     ar$pro   16 bits  Protocol type
     ar$shtl   8 bits  Type & length (TL) of source ATM number (q)
     ar$sstl   8 bits  Type & length (TL) of source ATM subaddress (r)
     ar$op    16 bits  Operation code (request, reply, or NAK)
     ar$spln   8 bits  Length of source protocol address (s)
     ar$thtl   8 bits  Type & length (TL) of target ATM number (x)
     ar$tstl   8 bits  Type & length (TL) of target ATM subaddress (y)
     ar$tpln   8 bits  Length of target protocol address (z)
     ar$sha   qoctets of source ATM number
     ar$ssa   roctets of source ATM subaddress
     ar$spa   soctets of source protocol address
     ar$tha   xoctets of target ATM number
     ar$tsa   yoctets of target ATM subaddress
     ar$tpa   zoctets of target protocol address

   Where:
     ar$hrd  -  assigned to ATM Forum address family and is
                19 decimal (0x0013) [4].

     ar$pro  -  see Assigned Numbers for protocol type number for
                the protocol using ATMARP. (IP is 0x0800).

     ar$shtl -  Type and length of source ATM number.  See
                Section 8.7.4 for TL encoding details.

     ar$sstl -  Type and length of source ATM subaddress.  See
                Section 8.7.4 for TL encoding details.

     ar$op   -  The operation type value (decimal):

                ATMARP_Request   = ARP_REQUEST   = 1
                ATMARP_Reply     = ARP_REPLY     = 2
                InATMARP_Request = InARP_REQUEST = 8
                InATMARP_Reply   = InARP_REPLY   = 9
                ATMARP_NAK       = ARP_NAK       = 10

     ar$spln -  length in octets of the source protocol address. Value
                range is 0 or 4 (decimal).  For IPv4 ar$spln is 4.

     ar$thtl -  Type and length of target ATM number.  See
                Section 8.7.4 for TL encoding details.

     ar$tstl -  Type and length of target ATM subaddress.  See
                Section 8.7.4 for TL encoding details.

     ar$tpln -  length in octets of the target protocol address. Value
                range is 0 or 4 (decimal).  For IPv4 ar$tpln is 4.

     ar$sha  -  source ATM number (E.164 or ATM Forum NSAPA)

     ar$ssa  -  source ATM subaddress (ATM Forum NSAPA)

     ar$spa  -  source protocol address

     ar$tha  -  target ATM number (E.164 or ATM Forum NSAPA)

     ar$tsa  -  target ATM subaddress (ATM Forum NSAPA)

     ar$tpa  -  target protocol address

8.7.2 Receiving Unknown ATMARP packets

   If an ATMARP client receives an ATMARP message with an operation code
   (ar$op) for which it is not coded to support, it MUST gracefully
   discard the message and continue normal operation.  An ATMARP client
   is NOT REQUIRED to return any message to the sender of the
   unsupported message.

8.7.3 TL, ATM Number, and ATM Subaddress Encoding

   The encoding of the 8-bit TL (type and length) fields in ATMARP and
   In_ATMARP packets is as follows:

     MSB   8     7     6     5     4     3     2     1   LSB
        +-----+-----+-----+-----+-----+-----+-----+-----+
        |  0  | 1/0 |   Octet length of address         |
        +-----+-----+-----+-----+-----+-----+-----+-----+

   Where:
     bit.8   (reserved) = 0  (for future use)

     bit.7   (type)     = 0  ATM Forum NSAPA format
                        = 1  E.164 format

     bit.6-1 (length)   = 6 bit unsigned octet length of address
                          (MSB = bit.6, LSB = bit.1)  Value
                          range is from 0 to 20 (decimal).

   ATM addresses, as defined by the ATM Forum UNI 3.1 signaling
   specification [9], include a "Calling Party Number Information
   Element" and a "Calling Party Subaddress Information Element".  These
   Information Elements (IEs) SHOULD map to ATMARP/InATMARP source ATM
   number and source ATM subaddress respectively.  Furthermore, ATM
   Forum defines a "Called Party Number Information Element" and a
   "Called Party Subaddress Information Element".  These IEs map to
   ATMARP/InATMARP target ATM number and target ATM subaddress,
   respectively.

   The ATM Forum defines three structures for the combined use of number
   and subaddress [9]:

                        ATM Number      ATM Subaddress
                      --------------    --------------
        Structure 1   ATM Forum NSAPA        null
        Structure 2       E.164              null
        Structure 3       E.164         ATM Forum NSAPA

   ATMARP and InATMARP requests and replies for ATM address structures 1
   and 2 MUST indicate a null or unknown ATM subaddress by setting the
   appropriate subaddress length to zero; i.e., ar$sstl.length = 0 or
   ar$tstl.length = 0, the corresponding type field (ar$sstl.type or
   ar$tstl.type) MUST be ignored and the physical space for the ATM
   subaddress buffer MUST not be allocated in the ATMARP packet.  For
   example, if ar$sstl.length=0, the storage for the source ATM
   subaddress is not allocated and the first byte of the source protocol
   address ar$spa follows immediately after the last byte of the source
   hardware address ar$sha in the packet.

   Null or unknown ATM addresses MUST be indicated by setting the
   appropriate address length to zero; i.e., ar$shtl.length and
   ar$thtl.length is zero and the corresponding type field (ar$sstl.type
   or ar$tstl.type) MUST be ignored and the physical space for the ATM
   address or ATM subaddress buffer MUST not be allocated in the ATMARP
   packet.

8.7.4 ATMARP_NAK Packet Format

   The ATMARP_NAK packet format is the same as the received
   ATMARP_Request packet format with the operation code set to ARP_NAK,
   i.e., the ATMARP_Request packet data is exactly copied (e.g., using
   bcopy) for transmission with the ATMARP_Request operation code
   changed to ARP_NAK value.

8.7.5 Variable Length Requirements for ATMARP Packets

   ATMARP and InATMARP packets are variable in length.

   A null or unknown source or target protocol address is indicated by
   the corresponding length set to zero: e.g., when ar$spln or ar$tpln
   is zero the physical space for the corresponding address structure
   MUST not be allocated in the packet.

   For backward compatibility with previous implementations, a null IPv4
   protocol address may be received with length = 4 and an allocated
   address in storage set to the value 0.0.0.0.  Receiving stations MUST
   be liberal in accepting this format of a null IPv4 address.  However,
   on transmitting an ATMARP or InATMARP packet, a null IPv4 address
   MUST only be indicated by the length set to zero and MUST have no
   storage allocated.

8.8 ATMARP/InATMARP Packet Encapsulation

   ATMARP and InATMARP packets are to be encoded in AAL5 PDUs using
   LLC/SNAP encapsulation.  The format of the AAL5 CPCS-SDU payload
   field for ATMARP/InATMARP PDUs is:

               Payload Format for ATMARP/InATMARP PDUs:
               +------------------------------+
               |        LLC 0xAA-AA-03        |
               +------------------------------+
               |        OUI 0x00-00-00        |
               +------------------------------+
               |     EtherType 0x08-06        |
               +------------------------------+
               |                              |
               |   ATMARP/InATMARP Packet     |
               |                              |
               +------------------------------+

   The LLC value of 0xAA-AA-03 (3 octets) indicates the presence of a
   SNAP header.

   The OUI value of 0x00-00-00 (3 octets) indicates that the following
   two-bytes is an EtherType.

   The EtherType value of 0x08-06 (2 octets) indicates ARP [4].

   The total size of the LLC/SNAP header is fixed at 8-octets.  This
   aligns the start of the ATMARP packet on a 64-bit boundary relative
   to the start of the AAL5 CPCS-SDU.

   The LLC/SNAP encapsulation for ATMARP/InATMARP presented here is
   consistent with the treatment of multiprotocol encapsulation of IP
   over ATM AAL5 as specified in [2] and in the format of ATMARP over
   IEEE 802 networks as specified in [5].

   Traditionally, address resolution requests are broadcast to all
   directly connected IP members within a LIS.  It is conceivable in the
   future that larger scaled ATM networks may handle ATMARP requests to
   destinations outside the originating LIS, perhaps even globally;
   issues raised by ATMARPing outside the LIS or by a global ATMARP
   mechanism are beyond the scope of this memo.

9.  IP BROADCAST ADDRESS

   ATM does not support broadcast addressing, therefore there are no
   mappings available from IP broadcast addresses to ATM broadcast
   services.  Note: this lack of mapping does not restrict members from
   transmitting or receiving IP datagrams specifying any of the four
   standard IP broadcast address forms as described in [8].  Members,
   upon receiving an IP broadcast or IP subnet broadcast for their LIS,
   MUST process the packet as if addressed to that station.

   This memo recognizes the future development of standards and
   implementations that will extend the operations as defined in this
   memo to provide an IP broadcast capability for use by the classical
   client.

10.  IP MULTICAST ADDRESS

   ATM does not directly support IP multicast address services,
   therefore there are no mappings available from IP multicast addresses
   to ATM multicast services.  Current IP multicast implementations
   (i.e., MBONE and IP tunneling, see [10]) will continue to operate
   over ATM based logical IP subnets if operated in the WAN
   configuration.

   This memo recognizes the future development of ATM multicast service
   addressing by the ATM Forum.  When available and widely implemented,
   the roll-over from the current IP multicast architecture to this new
   ATM architecture will be straightforward.

   This memo recognizes the future development of standards and
   implementations that will extend the operations as defined in this
   memo to provide an IP multicast capability for use by the classical
   client.

11.  SECURITY CONSIDERATIONS

   Not all of the security issues relating to IP over ATM are clearly
   understood at this time, due to the fluid state of ATM
   specifications, newness of the technology, and other factors.

   It is believed that ATM and IP facilities for authenticated call
   management, authenticated end-to-end communications, and data
   encryption will be needed in globally connected ATM networks.  Such
   future security facilities and their use by IP networks are beyond
   the scope of this memo.

   There are known security issues relating to host impersonation via
   the address resolution protocols used in the Internet [13].  No
   special security mechanisms have been added to the address resolution
   mechanism defined here for use with networks using IP over ATM.

12.  MIB SPECIFICATION

   Clients built to this specification MUST implement and provide a
   Management Information Base (MIB) as defined in "Definitions of
   Managed Objects for Classical IP and ARP Over ATM Using SMIv2" [18].

13.  OPEN ISSUES

   o   Automatic configuration of client ATM addresses via DHCP [15] or
       via ATM UNI 3.1 Interim Local Management Interface (ILMI)
       services would be a useful extended service addition to this
       document and should be addressed in a separate memo.

   o   ATMARP packets are not authenticated.  This is a potentially
       serious flaw in the overall system by allowing a mechanism by
       which corrupt information may be introduced into the server
       system.

14. REFERENCES

   [1] Piscitello, D., and J. Lawrence, "The Transmission of IP
       Datagrams over the SMDS Service", STD 52, RFC 1209, March 1991.

   [2] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
       Layer 5", RFC 1483, July 1993.

   [3] Plummer, D., "An Ethernet Address Resolution Protocol - or -
       Converting Network Protocol Addresses to 48.bit Ethernet
       Address for Transmission on Ethernet Hardware", STD 37, RFC
       826, November 1982.

   [4] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
       July 1992.

   [5] Postel, J., and J. Reynolds, "A Standard for the Transmission
       of IP Datagrams over IEEE 802 Networks", STD 43, RFC 1042,
       February 1988.

   [6] CCITT, "Draft Recommendation I.363", CCITT Study Group XVIII,
       Geneva, 19-29 January 1993.

   [7] CCITT, "Draft text for Q.93B", CCITT Study Group XI, 23 September
       - 2 October 1992.

   [8] Braden, R., "Requirements for Internet Hosts -- Communication
       Layers", STD 3, RFC 1122, October 1989.

   [9] ATM Forum, "ATM User-Network Interface (UNI) Specification
       Version 3.1.", ISBN 0-13-393828-X, Prentice-Hall, Inc., Upper
       Saddle River, NJ, 07458, September, 1994.

   [10] Deering, S., "Host Extensions for IP Multicasting", STD 5,
        RFC 1112, August 1989.

   [11] Colella, R., Gardner, E., and R. Callon, "Guidelines for OSI
        NSAP Allocation in the Internet", RFC 1237, July 1991.

   [12] Bradely, T., and C. Brown, "Inverse Address Resolution
        Protocol", RFC 1293, January 1992.

   [13] Bellovin, Steven M., "Security Problems in the TCP/IP Protocol
        Suite", ACM Computer Communications Review, Vol. 19, Issue 2,
        pp. 32-48, 1989.

   [14] Knowles, S., "IESG Advice from Experience with Path MTU
        Discovery", RFC 1435, March 1993.

   [15] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
        March 1997.

   [16] Kent C., and J. Mogul, "Fragmentation Considered Harmful",
        Proceedings of the ACM SIGCOMM '87 Workshop on Frontiers in
        Computer Communications Technology, August 1987.

   [17] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
        November 1990.

   [18] Green, M., Luciani, J., White, K., and T. Kuo, "Definitions of
        Managed Objects for Classical IP and ARP over ATM Using
        SMIv2", RFC 2320, April 1998.

   [19] ATM Forum, "ATM User-Network Interface (UNI) Specification
        Version 4.0", ATM Forum specfication af-sig-0061.000,
        ftp://ftp.atmforum.com/, July, 1996.

   [20] Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

15. AUTHORS' ADDRESSES

   Mark Laubach
   Com21, Inc.
   750 Tasman Drive
   Milpitas, CA 95035

   Phone: 408.953.9175
   FAX:   408.953.9299
   EMail: laubach@com21.com

   Joel Halpern
   Newbridge Networks, Inc.
   593 Herndon Parkway
   Herndon, VA  22070-5241

   Phone: 703.736.5954
   FAX:   703.736.5959
   EMail: jhalpern@Newbridge.com

APPENDIX A - Update Information

   This memo represents an update to RFC 1577 and RFC 1626.  The
   following changes are included in this memo:

   o   Pointer to Classical MIB I-D for setting of variables

   o   Single ATMARP server address to ATMARP server list, configurable
       via the MIB.

   o   RFC 1626 text replaces MTU section

   o   Client registration procedure from In_ATMARP to first
       ATMARP_Request

   o   Clarification of variable length ATMARP packet format

   o   Clarification of ARP_NAK packet format

   o   Clarification of InATMARP packet format for null IPv4 addresses

   o   Clarification on ATMARP registration and use of InATMARP_Reply
       for clients having more than one IP address in a LIS

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